Different Types of Introns

Since the discovery of introns
in pre-mRNA, other types of introns have also been
identified. We now know of 4 types of introns:
introns in tRNA genes, group I introns, group II
introns and pre-mRNA introns.
The tRNA introns are special because they are removed
by an enzyme which cuts
the RNA, after which other enzymes phosphorylate
(protein kinase) and religate the two halves of the
tRNA.

Group I introns are widespread
and present in mRNA, tRNA and rRNA in mitochondria,
chloroplasts and nuclear genomes of non-vertebrates,
in bacteriophages and in eubacterial genomes. All
group I introns share the same folding pattern. Group
I introns were studied by Tomas Cech, who found that
group I introns can be excised in the complete
absence of any proteins. This discovery led to the
concept of RNA enzymes and has
had great influence on our thinking on the evolution
of life on earth. Tomas Cech shared the Nobel prize
in 1989 with Sidney Altman who discovered that
another type of RNA, RNAse P could catalyse the
cleavage of the tRNA precursors.

Group II introns are found in
mitochndria and chloroplasts, but have also been
observed in cyanobacteria and
proteobacteria. All group II introns also share a
common folding pattern. As for group I introns, group
II introns can self-splice. In vivo, splicing of both
group I and II introns involves proteins. The
reaction mechanism of group II intron excision is
exactly the same as for pre-mRNA introns
and it is possible that pre-mRNA introns
and group II introns are evolutionally related. If
so, the snRNAs may originate from group II
introns.

Origin of Introns

The fact that introns are
present in genes has been used during evolution to
evolve alternative splicing as a
process to regulate gene expression. It has also made
it possible to build new genes by combining exons in
different combinations, so-called exon shuffling.
Several such genes are present in mammalian
organisms.

It is, however, less clear where introns come from in
the first place. Two hypotheses have been put
forward. According to the first hypothesis, the
exon theory,
introns were present in genes in the progenote, the
origin of all life forms.These introns may have been
self-splicing introns. After divergence of the three
major kingdoms, introns were lost from eubacteria and
archaebacteria to a
large extent.

The alternative hypothesis is that introns are a kind
of transposable elements which can jump in and out of
existing genes which did not necessarily have introns
from the beginning. A possible scenario would then be
that group II introns invaded nuclear genes after the
symbiotic event that created mitochondria and
chloroplasts. This view has gained support from the
fact that group II introns can jump into genes at
specific sites. Both hypotheses have received some
support from different kinds of observations and both
hypotheses may be partly true.